The ISSN (International Standard Serial Number) is an eight-digit number which identifies periodical publications as such, including electronic serials.

The ISSN takes the form of the acronym ISSN followed by two groups of four digits, separated by a hyphen. The eighth character is a control digit calculated according to a modulo 11 algorithm on the basis of the 7 preceding digits; this eighth control character may be an "X" if the result of the computing is equal to "10", in order to avoid any ambiguity.

The ISSN is linked to a standardized form of the title of the identified serial, known as the "key title", which repeats the title of the publication, qualifying it with additional elements in order to distinguish it from other publications having identical titles.

ISSN in the digital environment

ISSN are assigned since the 1970s and universally accepted within the print publishing world as a means of identifying the serials. Changes in the editorial practices linked to the development of Internet and of the digital technologies led the ISSN to evolve and to extend its scope to the whole continuing resources (i.e "publications that are issued over time with no predetermined conclusion") whatever their physical medium.

Consequently, and from the end of the 1990s, ISSN are also assigned to electronic serials (online, CD-ROM, DVD...) and to electronic "ongoing integrating resources" ( ie "continuing resources that are added to or changed by means of updates that do not remain discrete and are integrated into the whole, and has no predetermined conclusion") such as websites and databases.